How do you test a knock sensor?

1980 Audi 100, 120,000 Miles and counting.
I have learned about Fault codes and how to read them. I have three.
1. Tells me I need a new thermostat. I knew that.
2. Tells me the Full throttle switch doesn't work. I tested it with a
multimeter and it doesn't. Now I have to figure out how to replace the switch.
It seems to be mounted together with the idle switch. Anyone have experience
with this?
3. Tells me my Knock sensor is not working. Here I'm really stumped. How do
you test a knock sensor? I don't have a timing light so I'm hoping to do a test
with a multimeter.
Regards,
DPowell